1. Field of the Invention
The present invention relates to a semiconductor device such as a dynamic memory and a method of manufacturing the same.
2. Related Art of the Invention
It has conventionally been known that a leakage current generated in a memory cell of a dynamic memory adversely affects the charge retention characteristic of the memory cell. To prevent this, for example, in a semiconductor device disclosed in Japanese Laid-open Patent Application No. S62-2562, both of the source and drain diffusion regions are formed low in concentration in a transistor which is a portion of a memory cell where the leakage current is necessarily prevented. FIG. 4(a) shows the structure thereof. In the figure, reference numeral 41 represents a silicon substrate (first conductivity type), reference numeral 42 represents an isolation oxide film (LOCOS), reference numeral 43 represents a gate oxide film, reference numeral 44 represents a gate electrode, reference numeral 45 represents low-concentration source and drain diffusion regions (second conductivity type), reference numeral 46 represents an oxide film, reference numeral 47 represents high-concentration source and drain diffusion regions (second conductivity type), and reference numeral 48 represents a lower electrode of a cell capacitor. The low-concentration source and drain diffusion regions 45 correspond to the both of the surface and drain diffusion regions of a transistor which is a portion where the leakage current is necessarily prevented.
Examples of means for increasing the capacity of the cell include one as disclosed in Japanese Laid-open Patent Application No. H2-177359. FIG. 4(b) shows the structure thereof. Portions the same as those shown in FIG. 4(a) will be described by use of the same reference numerals. Reference numeral 41 represents a silicon substrate (first conductivity type). Reference numeral 42 represents an isolation oxide film (LOCOS). Reference numeral 43 represents a gate oxide film. Reference numeral 44 represents a gate electrode. Reference numeral 45 represents low-concentration source and drain diffusion regions (second conductivity type). Reference numeral 46 represents an oxide film. Reference numeral 48 represents a lower electrode of a cell capacitor. Reference numeral 49 represents a high-concentration diffusion region (first conductivity type). In one of the diffusion regions (the one connected to the cell capacitor) in the source and the drain, the high-concentration diffusion region 49 having the same conductivity type as the substrate is formed and the capacity of the cell is increased by use of the junction capacitance.
However, since devices has recently become denser as the degree of integration increases, it is necessary to decrease the depth of the diffusion regions in order to prevent a short channel effect of a switching transistor. Therefore, only reducing the concentration of both of the source and drain diffusion regions like in the prior arts degrades the junction characteristic because the surface of the semiconductor substrate is etched in forming the contact hole for connecting the diffusion layers and the cell capacitor, so that the leakage current increases.
Moreover, forming the high-concentration diffusion regions of the different conductivity type so as to adjoin the drain and source diffusion regions in order to increase the capacity of the cell also increases the leakage drain.
Thus, the conventional methods all present a problem that the charge retention characteristic is degraded.